Technical Field
The present invention is generally related to a photoresist composition used in the manufacture of integrated circuits (IC's). More particularly, the invention relates to a polycyclic chemically amplified polymer resist that has light absorbing properties at deep ultraviolet (DUV) wave lengths.
Trends in the electronics industry continually require IC's that are faster and consume less energy. To meet these specifications the IC must be of a high density having sub-micron feature dimensions. Conducting lines must be made thinner and placed closer together. Reducing the spacing between conducting lines results in a concomitant increase in the efficiency of the IC enabling a greater storage capacity and faster processing of information on a computer chip. To achieve thinner line widths and smaller feature sizes higher patterning resolution is necessary.
In the manufacture of IC elements a chemically amplified photoresist is applied on the surface of a silicon wafer. A latent photo lithographic image is formed on the resist by irradiating its surface through an overlying photomask containing the desired patterning information. The exposed areas of the resist undergo a chemical change with attendant changes in solubility. The patterned resist is then developed in a solvent that selectively removes either the exposed or unexposed regions of the resist exposing the underlying wafer. Accordingly, the resist produces positive or negative images of the mask, depending upon the selection of developer solvent. The exposed silicon wafer substrate material is removed or changed by an etching process leaving the desired pattern in a functional layer of the wafer. The remaining resist material functions as a barrier
Etching generally involves passing a gas through a chamber and ionizing the gas by applying a potential across two electrodes in the presence of the gas. The plasma containing the ionic species generated by the potential is used to etch a substrate placed in the chamber. The ionic species generated in the plasma are directed to the patterned substrate where they interact with the surface material forming volatile products that are removed from the surface. Reactive ion etching (RIE) provides well defined vertical sidewall profiles in the substrate as well as substrate to substrate etching uniformity. Because of these advantages, the reactive ion etching technique has become the standard in IC manufacture.
To achieve the finer feature sizes demanded by today's specifications, higher photo lithographic imaging resolution is necessary. Higher resolution is possible with shorter wave lengths of the source employed to irradiate the photoresist (DUV at 190 to 315 nm). However, the prior art photoresists such as the phenol-formaldehyde novolac polymers and the substituted styrenic polymers contain aromatic groups that inherently become increasingly absorptive as the wave length of light falls below about 300 nm. A drawback is that the radiation can not fully penetrate the lower portions of the resist. Consequently, the surface portions of the resist receive a much larger dose of radiation than the lower portions. When the irradiated pattern is developed, a tapered profile is formed. Accordingly, fine feature resolution can not be attained. To overcome these transparency deficiencies it has been suggested to decrease the thickness of the resist in the hopes of increasing the transparency characteristics of the resist. However, as is common in the polymer art, the enhancement of one property is usually accomplished at the expense of another. Because photoresist materials are generally organic in nature and the substrates utilized in the manufacture of IC's are typically inorganic (e.g., silicon), the photoresist material has an inherently higher etch rate than the substrate material when employing the RIE process. With thinner layers of resist materials employed to overcome the transparency problem, the resist materials were eroded away before the underlying substrate could be fully etched.
U.S. Pat. No. 5,625,020 to Breyta et al. discloses a chemically amplified resist composition including a polymer comprising the reaction product of hydroxstyrene with acrylate, methacrylate, or mixtures thereof. Acrylate polymers have been proposed to overcome the transparency drawbacks of the phenolic-based photoresists. However, the gain in transparency to shorter wave length UV is achieved at the expense of sacrificing the resists' resistance to RIE processes.
J. V. Crivello et al. (Chemically Amplified Electron-Beam Photoresists, Chem. Mater., 1996, 8, 376-381) describe a polymer blend comprising 20 weight percent of a free radically polymerized homopolymer of a norbornene monomer bearing acid labile groups and 80 weight percent of a homopolymer of 4-hydroxy-.alpha.-methylstyrene containing acid labile groups for use in electron-beam photoresists. As discussed supra, the increased absorbity (especially in high concentrations) of aromatic groups renders these compositions opaque and unusable for short wave length imaging radiation below 200 nm. The disclosed compositions are suitable only for electron-beam photoresists and can not be utilized for deep UV imaging (particularly not for 193 nm resists). Crivello et al. investigated blend compositions because they observed the oxygen plasma etch rate to be unacceptably high for free radically polymerized homopolymers of norbornene monomers bearing acid labile groups.
International Patent Application WO 97/33198 to The B. F. Goodrich Company discloses a chemically amplified photoresist composition comprising a polycyclic polymer containing repeating units having pendant acid labile groups. While a variety of other pendant groups are disclosed, no aromatic groups are included. In fact, the Background of the Invention states that "If deep UV transparency is desired (i.e., for 248 nm and particularly 193 nm wave length exposure), the polymer should contain a minimum of aromatic character."
Accordingly, there is still a need for a photoresist composition which is compatible with the general chemical amplification scheme and provides transparency to short wave length imaging radiation while being sufficiently resistant to a reactive ion etching processing environment.